Many furnace systems include a control system that includes sensors, relays and control valves that are used to safely start and operate the furnace system. In operation, the furnace receives a signal to start from a thermostat. The system starts an inducer blower that creates a negative pressure in a collector box of the furnace by drawing air from a combustion air inlet and outputting air through a flue outlet. The system senses the negative pressure and starts a burner assembly in a heat exchanger portion of the system by providing fuel to the burner assembly and igniting the fuel. A blower motor may then start, such that the blower motor receives air flow from a return air duct of a space and outputs the air to the heat exchanger portion of the system. The heated air is output via a supply air duct to heat the space.